Preparation and characterization of platinum nanoparticles immobilized in dihydrocinchonidine-modified montmorillonite and hectorite

A novel synthetic method is reported for the preparation of highly dispersed platinum nanoparticles immobilized in the swelling clay minerals Na-montmorillonite and Optigel (synthetic Na-hectorite), modified by cation exchange with dihydrocinchonidine hydrochloride (DHCd·HCl). For both smectites, X-ray diffraction (XRD) data confirmed intercalation of the chiral modifier between the clay mineral layers. After ion exchange, the ability of montmorillonite to undergo swelling in organic solvents was found to be superior to that of Optigel. The surface-modified smectites were impregnated with H2PtCl6 and the precursor was subsequently reduced with NaBH4. Transmission electron microscopy (TEM) images of Pt-montmorillonite (Pt-M) and Pt-Optigel (Pt-O) revealed the formation of ultrafine, monodispersed Pt nanoparticles, with mean crystallite sizes of 0.8 and 1.2 nm for Pt-M and Pt-O, respectively. Both samples proved to be mildly enantioselective catalysts for the asymmetric hydrogenation of ethyl pyruvate, leading to the predominant formation of (R)-ethyl lactate. The finding that the efficiency of Pt-M was affected by the dispersion medium through disaggregation suggested that access for the reactant to the active centres was rather limited. On the other hand, the improved enantiodifferentiation and the lack of a solvent effect for Pt-O indicated easier access for the ethyl pyruvate molecules to the active centres located mainly on the external surface sites of the clay mineral lamellae. For the above reaction, the occurrence of such external active sites is considered to be responsible for the improved catalytic performance, whereas the role of the Pt particle size is regarded as less significant.